High-reliability silver-bearing thick film conductors for automotive applications

The reliability of several soldered thick-film Pd/Ag and Pt/Ag conductors under thermal cycles is reported. Evaluations were made of the thermal cycle adhesion by wire peel tests, as well as the electrical reliability of soldered components. In the case of Pt/Ag, the failure mode with mounted components was at the conductor/solder interface. The results of the electrical reliability test generally agreed with peel test results. However, when components were soldered to Pd/Ag conductors, vertical cracks were often observed through the conductor at the edge of the solder fillet. These cracks did not correlate with failure modes from peel tests. The tendency to vertical cracking was significantly reduced with the use of a dense firing Pd/Ag composition. No vertical cracks were observed with the Pt/Ag conductor because of the much slower rate of Sn intermetallic formation compared with Pd/Ag conductors. The design windows for both Pd/Ag and Pt/Ag conductors are discussed from the circuit design point of view